Sensor arrangement and method for operating a sensor arrangement

ABSTRACT

A sensor arrangement comprises at least a first, a second, and a third light sensor. A three-dimensional framework comprises at least a first, a second, and a third connection means which are connected to the at least first, second, and third light sensor, respectively. The first, the second, and the third connection means are configured to align the at least first, second, and third light sensor along a first, second, and third face of a polyhedron-like volume, respectively, such that the sensor arrangement encloses the polyhedron-like volume. The invention also relates to a method for operating the sensor arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority under 35 U.S.C.§119 from U.S. Provisional Patent Application Ser. No. 61/946,951, filedon Mar. 3, 2014, and claims priority to European Patent Application No.14157910.2 filed on Mar. 5, 2014, both disclosures of which are herebyincorporated by reference in their entirety for all purposes.

This invention relates to a sensor arrangement and to a method foroperating a sensor arrangement.

BACKGROUND OF THE INVENTION

Generally, ambient light sensing is the ability to measure thebrightness of light incident on a surface. Ambient light sensors such aslux meters, however, strive to sense light in a somewhat directionallyinvariant manner. Typically, these sensors use a diffuser dome placedover the light sensitive surface, which removes all directionalinformation from ambient light signals. The sensor characteristictypically takes the form of a modified cosine response. In addition, theuse of a diffuser dome attenuates the response, making the ambient lightsensor less sensitive.

However, there are many applications of ambient light sensor that canbenefit from the ability to sense the direction of incident light. Suchapplications include solar tracking, e.g. for automated tracking ofsolar panels and controlling of automated window blinds in a modernhouse. Furthermore, directional ambient light sensors could be used forcontrolling a car's rear view mirror at night or dimming the headlightsin a car. In sun- or moonlit applications, directional ambient lightsensors could even be used as an inexpensive navigation method. Forexample, a remote controlled (R/C) boat would be able to autonoumslyfind its way back to shore if the boat accidently got out of range ofthe R/C transmitter. Even further advanced applications are possible,navigation of a car, for example.

The objective of this invention is to provide a sensor arrangement and amethod for operating a sensor arrangement that improves on directionallight sensing.

This object is solved by the subject matter of the independent claims.Further developments and embodiments relate to dependent claims.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a sensor arrangement comprisesat least a first, a second, and a third light sensor. Athree-dimensional framework comprises at least a first, a second, and athird connection means. These connection means are connected to the atleast first, second, and third light sensors, respectively. The first,the second, and the third connection means are configured to align theat least first, second, and third light sensor along faces of apolyhedron-like volume. In particular, the first light sensor is alignedalong a first face, the second light sensor is aligned along a secondface, and the third light sensor is aligned along a third face of thepolyhedron-like volume, respectively. The three-dimensional frameworkand, more particularly, the connection means, align the light sensorssuch that the sensor arrangement encloses the polyhedron-like volume.

In operation of the sensor arrangement, the individual light sensorsgenerate first, second, and third sensor signals as a response toincident light. As the respective light sensors are positioned alongdifferent faces of the polyhedron-like volume, the sensor signals aredepending on the three-dimensional structure of the polyhedron-likevolume. In fact, the sensor signals become dependent on the orientationof the face of the polyhedron-like volume that they are aligned to.Thus, the specific population of light sensors along faces of thepolyhedron-like volume determines a region of interest from which thelight sensors can detect light. The region of interest describes thefield of view of the sensor arrangement. For example, the sensor's candetect light from a certain region, from a hemisphere, or even from alldirections (omnidirectional sensing). In turn, the sensor signals areindicative of a direction or position of a light-source to be detected.

The proposed sensor arrangement uses multiple sensors placed in athree-dimensional geometry determined by the geometry of polyhedron-likevolume. It offers the ability of sensing the direction of incident lightand/or sensing light intensity of a light-source within the region ofinterest. Furthermore, if a light-source is limited to move and emitwithin only within that region of interest the sensing can be invariantto the direction of the incident light. This approach can be used tocreate a truly omnidirectional light sensor that does not need to use adiffusor.

According to another aspect of the invention, the first, the second, andthe third light sensor are aligned with their light-sensitive surfacesfacing away from the polyhedron-like volume. In this context, a surfacecan be assigned to the polyhedron-like volume for illustration. Thelight-sensitive surfaces of the light sensors are then facing away fromthe surface of the polyhedron-like volume.

Furthermore, the first, the second, and the third light sensor areconfigured to detect light from within a continuous region of interestdepending on the polyhedron-like volume. A continuous region of interestcan be established by aligning the respective light sensors in a waythat their individual fields of view are at least partially overlappingor border on each other. In this way, light is only detected from withinthe continuous region of interest.

The detected intensity of light from the continuous region of interestmay be compared with a predetermined value expected to be emitted fromthe light-source. This feature can be used for implementing a trackingmechanism based on sensor signals generated by the sensor arrangement.In fact, the light sensors typically detect different light intensities,which should sum up to yield the predetermined value. The position ofthe light-source can be determined from such relative light intensities.

According to another aspect of the invention, the at least first,second, and third light sensor are aligned with respect to athree-dimensional coordinate system. Generally, the three-dimensionalcoordinate system does not have to be aligned with the geometry of thepolyhedron-like volume. In order to reduce computational effort forprocessing the sensor signals, however, it may be useful to align thethree-dimensional coordinate system with respect to the polyhedron-likevolume.

Furthermore, the at least first, second, and third light sensor have asensor response characteristic which renders their sensor signalsproportional to a directional cosine in the three-dimensional coordinatesystem (cosine response).

According to another aspect of the invention, the at least first,second, and third light sensors comprise a photodiode, a ComplementaryMetal Oxide Semiconductor (CMOS) light detector, and/or a CCD,respectively. In another embodiment, photodiodes sensitive to infraredlight are implemented as these can reside behind optically opaquematerial so that they are not visible when built into a device.

According to another aspect of the invention, the at least first,second, and third light sensors comprise a colour sensor, respectively.The colour sensor can be a dedicated colour sensor, which byconstruction is sensitive to different wavelengths of light. However,the colour sensor may comprise several sensors of the same type, e.g.photodiodes, which are provided with one or more filters of differenttransmission characteristics to render their sensor signals dependent oncolour.

According to another aspect of the invention, the three-dimensionalframework comprises the polyhedron-like volume. In particular, thethree-dimensional framework comprises a solid body having thepolyhedron-like volume. The first, second, and third connection meansthen constitute the first, second, and third faces of thepolyhedron-like volume. Furthermore, the connection means are configuredto provide electrical terminals to the at least first, second, and thirdlight sensor.

In this case, when the three-dimensional framework has thepolyhedron-like volume, the light sensors can be directly connected tothe different faces of the volume. In a certain sense, thethree-dimensional framework constitutes a base to which the lightsensors can be connected and aligned.

According to another aspect of the invention, the three-dimensionalframework comprises a grid having an envelope comprising thepolyhedron-like volume. If a three-dimensional framework is designed asa grid, the first, second, and third connection means could be designedas interconnections having a central portion from which the connectionmeans spread out to connect the individual light sensors.

Generally, the light sensors are mounted on an integrated circuit boardand these circuit boards could be connected to each other so as to alsocomprise the polyhedron-like volume.

According to another aspect of the invention, the polyhedron-like volumecomprises a four-faceted pyramid or a frustum thereof. In particular,the polyhedron-like volume comprises a pyramid with a square base or afrustum thereof. Then, the at least first, second, and third lightsensors are aligned along the at least first, second, and third face ofthe four-facetted pyramid or the frustum thereof. Furthermore, a fourthlight sensor is aligned to a fourth face on the four-faceted pyramid orthe frustum thereof. A frustum is the portion of a volume, e.g. a coneor pyramid that lies between two parallel planes cutting it.

According to another aspect of the invention, the polyhedron-like volumecomprises a cube. The first, second, and third light sensors are alignedalong the at least first, second, and third face of the cube.Furthermore, the fourth light sensor is aligned along the fourth face ofthe cube.

According to another aspect of the invention, a fifth light sensorand/or a sixth light sensor is/are aligned a fourth and/or a sixth faceof the cube.

Using five light sensors, the direction of light can be sensed over ahemisphere. Furthermore, by using six sensors the direction of lightincident on the sensor arrangement can be sensed over a full sphere(light incident from any direction in three-dimensional space). The samegeometrical setup or sensor arrangement also allows omnidirectional,i.e. truly directional invariant, sensing over a full hemisphere or fullsphere.

A further aspect of the invention relates to a method for operating asensor arrangement comprising at least a first, a second, and a thirdlight sensor wherein each light sensor is aligned along a face of apolyhedron-like volume, respectively, so that the sensor arrangementencloses the polyhedron-like volume. The method comprises the followingsteps:

Firstly, collect from the at least first, second, and third light sensorrespective sensors signals depending on a light source to be detected.Then, determine from the collected sensor signals projections onto theprincipal axes of a three-dimensional coordinate system. Finally,determine from the determined projections a direction of the lightsource to be detected with respect to the three-dimensional coordinatesystem.

The proposed method makes use of multiple sensors placed in athree-dimensional geometry determined by the polyhedron-like volume toprovide directional sensing. In particular, it offers the ability ofsensing the direction of incident light and/or sensing light intensityof a light-source within a region of interest determined by thealignment of the light sensors. Furthermore, if a light-source islimited to move and emit within only within that region of interest thesensing can be invariant to the direction of the incident light. Thisapproach can be used to create a truly omnidirectional light sensor thatdoes not need to use a diffusor.

According to another aspect of the invention, the method furthercomprises the step of aligning the at least first, second, and thirdlight sensors. The alignment is defined with respect to the principalaxes of the three-dimensional coordinate system, respectively, i.e. thesensor signals relate to principal axes.

According to another step of the invention, the step of aligning the atleast first, second, and third light sensors involves defining acontinuous region of interest into which the light source to be detectedemits a predetermined amount of light.

In this way, light is only detected from within the continuous region ofinterest. The detected intensity of light from the continuous region ofinterest may be compared with a predetermined amount expected to beemitted from the light-source. This feature can be used for tracking theposition of the light-source based on sensor signals generated by thesensor arrangement. In fact, the light sensors typically detectdifferent light intensities, which should sum up to yield thepredetermined value. The position of the light-source can be determinedfrom such relative light intensities.

According to another aspect of the invention, a relative brightness isdetermined from the sensor signals. This is determined by at least twoalternatives or combinations thereof. According to a first alternative,the square root of the sum of squared sensor signals is taken. Accordingto a second alternative, the square root of the sum of the squareddifference signals is taken. The difference signals depend on the sensorsignals and may account for ambient light as well.

According to another aspect of the invention, the steps of collectingsensor signals, determining the projections onto the principal axis anddetermining the direction of the light source to be detecting iscontinuously repeated so as to track the direction of the light sourcewithin the three-dimensional coordinate system.

In the following, the principle presented above will be described inmore detail with respect to drawings in which exemplary embodiments arepresented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C show exemplary embodiments of a sensor arrangementaccording to the principle presented,

FIG. 2 shows an exemplary embodiment of the method for operating asensor arrangement according to the principle presented, and

FIGS. 3A-3B show an exemplary test result of an application using asensor arrangement according to the principle presented.

DETAILED DESCRIPTION

FIGS. 1A, 1B, 1C show exemplary embodiments of a sensor arrangementaccording to the principle presented. Generally, the sensor arrangementcomprises a polyhedron-like volume with individual light sensorsconnected and aligned along faces of the said volume.

Generally, the polyhedron-like volume can have the shape of anypolyhedron or similar structures, e.g. a frustum of a polyhedron.According to a geometrical defintion (seehttp://en.wikipedia.org/wiki/Polyhedron) a polyhedron is athree-dimensional shape that is made up of a finite number of polygonalfaces which are parts of planes; the faces meet in pairs along edgeswhich are straight-line segments, and the edges meet in points calledvertices. Cubes, prisms and pyramids are examples of polyhedra. Thepolyhedron surrounds a bounded volume in three-dimensional space;sometimes this interior volume is considered to be part of thepolyhedron, sometimes only the surface is considered, and occasionallyonly the skeleton of edges. FIG. 1A shows four-faceted pyramid, FIG. 1Bshows a frustum thereof, and FIG. 1C shows a cube as examples ofpolyhedral-like volumes.

The polyhedron-like volume can be thought of as a reference framework.In the exemplary embodiments shown in FIGS. 1A, 1B, and 1C thisframework is a solid body to which the light sensors are connected. Thebody has the polyhedron-like volume and comprises faces which constituteconnection means to which the light sensors are (mechanically andelectrically) connected. Furthermore, the faces may provide electricalterminals necessary to operate the sensor arrangement.

Alternatively, the framework can have a grid or skeleton design but itsenvelope comprises or encloses the polyhedron-like volume (not shown).In this design the framework is no solid body and the connection meansrather are parts of the grid.

The light sensors are connected to the connection means or to faces ofthe polyhedron-like volume (indicated as spheres in the drawings). Thelight sensors comprise a photodiode, a Complementary Metal OxideSemiconductor (CMOS) light detector, and/or a CCD, respectively.Photodiodes may be sensitive to infrared light. These can reside behindoptically opaque material so that they are not visible.

The light sensors typically have a direction dependent detectioncharacteristic. The term “cosine response” relates to the fact that thelight sensors generate a sensor signal S which depends on a directioncosine when light is incident under some angle θ, i.e. S cc cos θ. Theterms response and sensor signal will be used as equivalentshereinafter.

FIG. 2 shows an exemplary embodiment of the method for operating asensor arrangement according to the principle presented. The sensorarrangement can be used to track the position of a (point-like)light-source, such as the sun, and to determine the intensity of thatlight-source by using the exemplary method steps explained below. Forexample, under exposure to direct sunlight, i.e. clear skies, theresponse of light sensors can be evaluated to not only find the absoluteintensity, but also the direction of the sun.

In this context, the exemplary embodiment is based on a sensor cube,e.g. with five faces of the cube are populated by respective lightsensors, in particular by ambient light sensors, which all have a cosineresponse. The cube is depicted on the right side of the drawing in whicha first, a second, and a third light sensor are visible. A fourth and afifth light sensor are present but are connected to faces of the cubenot visible in the drawing.

However, the sensor arrangement can be complemented by a sixth lightsensor which can be connected to the remaining face of the cube andalong −a_(z). Whereas five light sensors on the cube can be used forsensing over a hemisphere, the addition of the sixth sensor allows fortruly omnidirectional sensing. In the following both embodiments will bedescribed in parallel and changes in the method will be highlighted.

The cube defines a three-dimensional coordinate system with principalaxes x, y, z, for example a Cartesian coordinate system (see left sideof FIG. 2). The coordinate system is described by the unit vectorsa_(x), a_(y), and a_(z), respectively. The light sensors are connectedto the cube with their light sensitive surfaces aligned along a_(x),−a_(x), a_(y), −a_(y) and a_(z) and oriented away from the surface ofthe cube. The sixth light sensor can be aligned along −a_(z). In otherwords with the fife sensor cube one sensor, for example, the fifth lightsensor, is oriented along the z-axis but not in the opposite direction.With the six sensor cube both directions are occupied.

For easier reference the light sensors will be referred to by theircorresponding unit vectors, e.g. first light sensor a_(x), second lightsensor −a_(x), third light sensor a_(y), fourth light sensor −a_(y), andfifth light sensor a_(z). The sixth sensor, if present, will be referredto as −a_(z).

For position determination or tracking of the sun it can be advisable toalign the light sensors with respect to a fixed reference. Thisreference can be given by earth's magnetic field, for example.Hereinafter X⁺ denotes north, X⁻ south, Y⁺ west, and Y⁻ east. The z-axisis aligned facing upward and downward, which will be denoted as Z⁺ andZ⁻ hereinafter. In the following it is assumed that the first lightsensor is aligned to north, the first light sensor is aligned north X+,the second light sensor X− is aligned south, the third light sensor isaligned west Y+, the fourth light sensor is aligned east Y−, and thefifth light sensor is aligned up. The sixth light sensor, if present, isaligned down.

After aligning the sensor arrangement in a next step the method foroperating a sensor arrangement is initialized by evaluating the lightsensors, i.e. by pairwise comparing the sensor signals Sx+, Sx−, Sy+,and Sy− of opposing sensors (i.e. a_(x) vs. −a_(x), then a_(y) vs.−a_(y)). The light sensor that has larger response is chosen over itsopposing one for further processing. This selection determines in whichquadrant (or in the case of having a sixth sensor, which octant) of thecoordinate system the light-source is positioned. In the coordinatesystem defined above there are four such quadrants, abbreviated asX⁺/Y⁺, X⁺/Y⁻, X⁻/Y⁺ and X⁻/Y⁻. In the case of six light sensors thepairwise comparing of the sensor signals also includes the opposingsensors a_(z) vs. −a_(z)) in order to determine up/down orientation.

In this exemplary method, once the quadrant, and eventually up/down, hasbeen determined the sensor signals of the remaining three sensors (i.e.the two a_(x) and a_(y) light sensors having the larger response, alongwith the a_(z) or −a_(z) light sensor) are selected and their sensorsignals processed. The processing involves finding the length of theposition vector V. For example, the light-source to be detected is thesun and its position is given by the position vector V having thecoordinates x₀, y₀, and z₀ or in terms of angles, angle α, β, and γ,which are given as the angles between the position vector and the x, y,and z axes, respectively (see drawing on the left side).

An arbitrary direction vector with coordinates x, y, z has a length l of

l=√{square root over (x ² +y ² +z ²)}.

The components of position vector V can be found by means of the scalardot product or by projection onto the the unit vectors a_(x), a_(y), anda_(z), respectively. By definition the unit vectors have length one. Ingeneral terms the position vector V is given as

V=V _(x) ·a _(x) +V _(y) ·a _(y) +V _(z) ·a _(z),

wherein V_(x), V_(y), V_(z) denote the vector components or magnitudesalong the x, y, and z axes, respectively. In fact, V_(x), V_(y), V_(z)are the projections of V onto the unit vectors a_(x), a_(y), and a_(z),respectively. This means that

V _(x) =V·a _(x) ,V _(y) =V·a _(y), and V _(z) =V·a _(z).

The scalar dot product “·” can also be evaluated by taking the productof a vector's magnitude or length. For example,V_(x)=V·a_(x)=|V|·|a_(x)| cos(α)=|V| cos(α), since |a_(x)|=1. Similarly,V_(y)=|V| cos(β) and V_(z)=|V|cos(γ).

For illustration purposes it is now assumed that three light sensorspointed along X⁺, Y⁺, and Z⁺ have been selected. The light sensors (e.g.photodiodes) have a cosine angular response. For example, light incidenton the sensor arrangement under vector V making an angle θ_(x) with asurface normal of the light sensor pointed in the X⁺ direction willinduce a response that is proportional to V·cos(α)=V·a_(x). If, forexample, all three light sensors pointed along X⁺, Y⁺, and Z⁺ arealigned so that their fields of view are illuminated by the impinginglight, then their respective responses will allow for measuring both thedirection and the intensity of the incident light vector V, assumingeither are matched, or can be matched by calibration, and have equal orknown responsitivity to the light.

The sensor response S_(z) of the a_(z) light sensor is proportional tothe cosine of the sun's zenith angle γ, i.e. the angle of positionvector V with the z axes (where the z axis is assumed to point upwards,e.g. towards the sky). It can also be shown that the sum of sensorsignals of all light sensors should meet the following constraint:

cos²(α)+cos²(β)+cos²(γ)=K ²,

where K is the number of counts resulting from any one of the lightsensors pointing directly at the sun. In FIG. 2, K is the length of thedirection vector pointing towards the sun. Please note that the term“cosine response” used above indicates that the sensor signals Sx, Sy,Sz are given as

S _(x) ^(±)∝cos(α),S _(y) ^(±)∝cos(β),S _(z) ^(±)∝cos(γ).

The strength of the sensor signals Sx, Sy, Sz can be measured in termsof counts. Let the sensor signals Sx, Sy, Sz (in counts) of the a_(x),a_(y), and a_(z) light sensor be given by X⁺ _(c), Y⁺ _(c), and Z⁺ _(c),. . . , respectively. Then the relative “brightness of the sun” (incounts) can be found as

K=√{square root over (X _(c) ² +Y _(c) ² +Z _(c) ²)}.

In other words without having to track the sun, e.g. by moving the wholesensor arrangement, the brightness of the sun can be monitored using theequation above. In addition, the sensor signals can be used to find thezenith angle γ as

$\gamma - {{arcos}\left( \frac{Z_{c}}{K} \right)}$

or the elevation angle EA=90°−γ.

Next, the azimuth angle AZ of the light source, e.g. the sun, can bedetermined as follows. Here X⁺ _(c) and X⁻ _(c) are the counts of theNorth and South facing detectors respectively, and Y⁺ _(c) and Y⁻ _(c)are the counts of the West and East facing sensors respectively, and letX_(c)′=max(X⁺ _(c),X⁻ _(c)) and Y_(c)′=max(Y⁺ _(c),Y⁻ _(c)), whereinmax( ) denotes the maximum. Then

δ=arctan(Y _(c) ′/X _(c)′),

withAZ=−δ if in quadrant where (X⁺ _(c)>X⁻ _(c) and Y⁺ _(c)>Y⁻ _(c))AZ=180°+δ if in quadrant where (X⁺ _(c)<X⁻ _(c) and Y⁺ _(c)>Y⁻ _(c)AZ=180°−δ if in quadrant where (X⁺ _(c)>−Y_(c) and Y⁺ _(c)<Y⁻ _(c)AZ=+δ if in quadrant where (X⁺ _(c)>−Y_(c) and +Y_(c)>Y⁻ _(c)).

Here, the definition of azimuth angle AZ is the conventional system of:North=0°, East=90°, South=180° and West=270°. Note that in the northernhemisphere, it will generally be the case that 90°≦AZ≦270°. For trackingthe position of the light-source the above mentioned steps arecontinuously repeated and the resulting coordinates saved.

Instead of the above described initial pairwise selection of lightsensors, it might be better to introduce difference signals such asΔX_(c)=(X⁺ _(c)−X⁻ _(c)), ΔY_(c)=(Y⁺ _(c)−Y⁻ _(c)), and ΔZ_(c)=(Z⁺_(c)−Z⁻ _(c)). This would help subtract out non-sunlight relatedbackground (ambient light), and would simplify the formulas. Forexample, the relative “brightness of the sun” (in counts) can be foundas

K=√{square root over (ΔX _(c) ² +ΔY _(c) ² ΔZ _(c) ²)}.

Now we can find the zenith angle γ, as

${\gamma = {{arcos}\left( {\Delta \frac{Z_{c}}{K}} \right)}},$

, and the elevation angle EA=90°−γ. Then δ=arctan(−ΔY_(c)/ΔX_(c). Incomputer programming the arctan function is evaluated using the a tan 2function which accepts two signed arguments in order to always evaluatethe arctangent within the proper quadrant, e.g. AZ=degrees (a tan 2(−ΔY_(c), ΔX_(c))).

Then the sun should never directly illuminate the light sensor facing tothe north, as long as the sensor arrangement is used in the northernhemisphere. Correspondingly the sun should never directly illuminate thelight sensor facing to the south, as long as the sensor arrangement isused in the southern hemisphere. Generally, that light sensor could beomitted.

The above introduced method can be extended to other geometries than acube. Other geometries besides “cubes” are lower and higher orderpolyhedrons (see also FIGS. 1A-C). In particular, three or four facetedpyramids as well as pyramids with the apex lopped off (frustum), such asshown in FIGS. 1A-C could be populated with light sensors placed atsome, e.g. excluding the bottom side or all faces. In this way thesensors point more upward towards the sky they are trying to monitor.Taking the cosine response on each light sensor as the projection onto agiven position vector V, in a direction orthogonal to the face of thesensor, the concept of finding the direction of a light-source such asthe sun or other pseudo point source can be generalized.

Consider a number of N light sensors, and the i'th light sensor bedescribed by a direction vector a_(i)a_(x)+b_(i)a_(y)+c_(i)a_(x), where1≦i≦N. Further assume all light sensors have the same sensitivity andthat the sensor signal or response (in number of counts) of each lightsensor is given by R_(i). The unit vectors are no longer mutuallyorthogonal, and the net normalized x,y, and z components need tonormalized by summing the individual responses of all light sensors anddividing that summed response by the projection of all of the lightsensors onto the x,y and z axes. Let the final unit direction vector beV_(d)=Xa_(x)+Ya_(y)+Za_(z), where √{square root over (X²+Y²+Z²)}=1. Theneach light sensor (with normal vector given by V_(i), where 1≦i≦N, foran N detector setup) will in general have a projection onto all threeaxes, which can be described by (V_(i))_(x), (V_(i))_(y) and(V_(i))_(z), where (V_(i))_(x)=V_(i)·a_(x), (V_(i))_(y)=V_(i)·a_(y) etc.In order to find the net x, y and z responses from multiple detectors,the responses need to be normalized as well.

FIGS. 3A and 3B show an exemplary test result of an application using asensor arrangement according to the principle presented. For trackingthe position of the light-source the above mentioned steps arecontinuously repeated and the resulting coordinates saved. Then, in anext step it is possible to map to other coordinate systems, for examplegeographical systems. Also, knowing the date and the latitude/longitudecoordinates of the site, one can also compute time from the position ofthe sun. Conversely, knowing the date and time and latitude/longitude,thus, being able to compute the sun's absolute position, one could usethis sensor arrangement as a compass by inverting the calculations.

FIG. 3A shows the tracking result using a prototype sensor arrangementusing five light sensors in a cube arrangement as introduced above. Thearrangement also contained a bubble level and a compass to orient thecube to the cardinal direction points. The sensor cube was placedunderneath a tinted bubble to allow the light sensors to operate indirect sunlight without saturating. The five light sensors could beinterfaced via the connection means to an I²C multiplexor board which isin turn interfaced to a “PICkit Serial” module that controlled the I²Cmultiplexor board using, for example, a Visual Basic program to sampleeach light sensor in the cube in sequence and store the readings.

The whole setup was connected via a USB cable to a laptop computerinside a car to collect the tracking data. FIG. 3A shows the dataplotted in an x, y representation while the car was driven around. Theresult shows the directional trajectory of the car in the x, y plane.For comparison FIG. 3B shows the actual trajectory on a map. This provesthat the sensor arrangement is capable of accurate directional sensing.The fact that the two trajectories do not meet in the start and endpoint is due to the fact that the car was driven with changing speed.However, this effect can be accounted for and only occurs in movingreference systems. For stationary tracking this is no issue.

One can also determine cloudy vs. sunny conditions by examining thesensor outputs, for example by tracking the intensity of thelight-source with time. Furthermore, by using color sensors, the sensorscould also detect color temperature and other data which could providefurther information such as partial clouded conditions, smog conditions,haze etc. Presence of haze, thin clouds and other scattering conditionscould also be ascertained from the sensor information by looking atclues such as the ratio of X⁺ _(c) to X⁻ _(c) e.g. If, for instance, X⁻_(c) is facing south and the unit is in the northern hemisphere, then X⁻_(c) should be significantly larger that X⁺ _(c) in direct sunlight.However if it is very hazy, then (X⁻ _(c))/(X⁺ _(c)) may be onlyslightly larger than one.

Moreover, one could be able to sense diffuse light levels in a mannersimilar to a diffuser dome sensor used in most lux meters, as mentionedin the beginning, by sensing a signal proportional to √{square root over((X⁻ _(c))²+(X⁺ _(c))²+(Y⁻ _(c))²+(Y⁺ _(c))²+(Z⁺ _(c))²)}{square rootover ((X⁻ _(c))²+(X⁺ _(c))²+(Y⁻ _(c))²+(Y⁺ _(c))²+(Z⁺ _(c))²)}{squareroot over ((X⁻ _(c))²+(X⁺ _(c))²+(Y⁻ _(c))²+(Y⁺ _(c))²+(Z⁺_(c))²)}{square root over ((X⁻ _(c))²+(X⁺ _(c))²+(Y⁻ _(c))²+(Y⁺_(c))²+(Z⁺ _(c))²)}{square root over ((X⁻ _(c))²+(X⁺ _(c))²+(Y⁻_(c))²+(Y⁺ _(c))²+(Z⁺ _(c))²)}.

In order for the system to function best, the sensor cube could beplaced on a flat black surface to minimize effects of light picked upfrom the environment. The sensor might also be mounted in a shallow boxto assure that the sensor only receives significant light for pointsABOVE the horizon (the “horizon” will be affected by presence of nearbytrees, buildings etc.).

In another embodiment (not shown) the sensor arrangement is used for adiffuse sky detector, detecting, e.g. cloudy or foggy skies verses sunnyskies. Let D be a “diffusivity” index which increases for diffuse (e.g.cloudy) skies, and is near unity for sunny (non-diffuse) sky conditions.Assuming a sensor arrangement of five light sensors, then:

${D = \frac{\sqrt{\left( X_{c}^{-} \right)^{2} + \left( X_{c}^{+} \right)^{2} + \left( Y_{c}^{-} \right)^{2} + \left( Y_{c}^{+} \right)^{2}}}{\sqrt{\left( {X_{c}^{-} - X_{c}^{+}} \right)^{2} - \left( {Y_{c}^{-} - Y_{c}^{+}} \right)^{2}}}},$

wherein the diffusivity index D will approach unity in a non-diffuse(e.g. direct sunlight) environment, but will grow large in diffuselight. For example, in direct sunlight, assuming that X⁻ _(c) and Y⁺_(c) might be exposed to direct sunlight, whereas X⁺ _(c) and Y⁻ _(c)are shadowed. This would make the denominator term large since thedifferences (X⁻ _(c)−X⁺ _(c)) and (Y⁻ _(c)−Y⁺ _(c)) will be large, whichshould make D small. (I.e. a small Diffusitivity index indicating thatthe light is direct instead of diffuse).

However, on a very cloudy day the diffuse light would scatter ordistribute light in all directions such that the (X⁻ _(c), X⁺ _(c)) and(Y⁻ _(c), Y⁺ _(c)) light sensor pairs would receive almost equal amountsof light making the differences (X⁻ _(c)−X⁺ _(c)) and (Y⁻ _(c)−Y⁺ _(c))very small, which makes the denominator small, thus making thediffusivity index D large, indicating diffuse light.

We claim:
 1. A sensor arrangement comprising: at least a first, asecond, and a third light sensor, a three-dimensional frameworkcomprising at least a first, a second, and a third connection meansconnected to the at least first, second, and third light sensor,respectively, and the first, the second, and the third connection meansbeing configured to align the at least first, second, and third lightsensor along a first, second, and third face of a polyhedron-likevolume, respectively, such that the sensor arrangement encloses thepolyhedron-like volume.
 2. The sensor arrangement according to claim 1,wherein the first, the second, and the third light sensor are alignedwith their light-sensitive surfaces facing away from the polyhedron-likevolume, and are configured to detect light from within a continuousregion of interest depending on the polyhedron-like volume.
 3. Thesensor arrangement according to claim 1, wherein the at least first,second, and third light sensor are aligned with respect to athree-dimensional coordinate system, and have a sensor responsecharacteristic proportional to a direction cosine in thethree-dimensional coordinate system.
 4. The sensor arrangement accordingto claim 1, wherein the at least first, second, and third light sensorscomprise a photo-diode, a CMOS light detector, and/or a CCD,respectively.
 5. The sensor arrangement according to claim 1, whereinthe at least first, second, and third light sensors comprise a colorsensor, respectively.
 6. The sensor arrangement according to claim 1,wherein the three-dimensional framework comprises the polyhedron-likevolume, in particular, comprises a solid body having the polyhedron-likevolume, the first, second, and third connection means constitute thefirst, second, and third face of the polyhedron-like volume, and theconnection means are configured to provide electrical terminals to theat least first, second, and third light sensor.
 7. The sensorarrangement according to claim 1, wherein the three-dimensionalframework comprises a grid having an envelope comprising thepolyhedron-like volume.
 8. The sensor arrangement according to claim 1,wherein the polyhedron-like volume comprises a four-faceted pyramid or afrustum thereof, in particular a pyramid with a square base or a frustumthereof, the first, second, and third light sensors are aligned alongthe first, second, and third face of the four-faceted pyramid or thefrustum thereof, and a fourth light sensor is aligned to a fourth faceof the four-faceted pyramid or the frustum thereof.
 9. The sensorarrangement according to claim 1, wherein the polyhedron-like volumecomprises a cube, the first, second, and third light sensors are alignedalong the first, second, and third face of the cube, and the fourthlight sensor is aligned along the fourth face of the cube.
 10. Thesensor arrangement according to claim 9, wherein a fifth light sensorand/or a sixth light sensor is/are aligned along a fourth and/or a sixthface of the cube.
 11. A method for operating a sensor arrangementcomprising at least a first, a second, and a third light sensor whereineach light sensor is aligned along a face of a polyhedron-like volume,respectively, so that the sensor arrangement encloses thepolyhedron-like volume, the method comprising the steps of: collectingfrom the at least first, second, and third light sensor respectivesensor signals depending on a light source to be detected, determinefrom the collected sensor signals projections onto the principal axes ofa three-dimensional coordinate system, and determine from the determinedprojections a direction of the light source to be detected with respectto the three-dimensional coordinate system.
 12. The method according toclaim 11, comprising the step of aligning the at least first, second,and third light sensors with respect to the principal axes of thethree-dimensional coordinate system, respectively.
 13. The methodaccording to claim 11, wherein the step of aligning the at least first,second, and third light sensors further involves defining a continuousregion of interest into which the light source to be detected emits apredetermined amount of light.
 14. The method according to claim 11,wherein from the sensor signals a relative brightness of the lightsource to be detected is determined by taking the square root of the sumof squared sensor signals, or by taking the square root of the sum ofsquared difference signals, wherein the difference signals depend on thesensor signals.
 15. The method according to claim 11, wherein the stepsof collecting sensor signals, determining the projections onto theprincipal axes and determining the direction of the light source to bedetected is continuously repeated so as to track the direction of thelight source within the three-dimensional coordinate system.